1. The present invention relates to a method for producing a steel sheet having super deep drawability, more particularly, to a method for producing a cold rolled steel sheet having excellent secondary workability as well as good chemical treatability.
2. Description of Prior Art
There are two general categories of steel sheet having super deep drawability: Ti killed steel sheet such as that described in U.S. Pat. No. 3,522,110, and Nb killed steel sheet such as that described in U.S. Pat. No. 3,761,324 or 3,876,390.
In this connection, as it has become easily possible to reduce the C content of Nb killed steel to the level of C&lt;50 ppm, there have been recent reports in the literature of the feasibility of producing Nb killed steel sheet with low C and Nb contents. On the presumption that the steel sheet is to have a very low C content, Ti or Nb, both of which have a strong tendency to form carbide and nitride, is added to obtain a steel sheet containing almost no interstitial elements such as C or N. There is thus the advantage that a steel sheet product of about the same quality can be obtained using either continuous or box annealing. In the case where the steel sheet is produced using the continuous anneal, however, there is encountered certain disadvantages as discussed in the following.
In the case of Ti killed steel, there is the disadvantage that secondary work cracking tends to occur. In particular, when, with the aim of obtaining a high quality steel, Ti is added to the steel at more than an equivalent amount with respect to C or N, an increase in the P content will increase risk of the secondary work cracking. Further, there is a disadvantage that addition of P will degrade the r value.
In addition, in the production of a steel sheet coated with alloyed zinc by means of the Sendzimir continuous molten zinc coating process, which constitutes one type of continuous annealing, the alloying proceeds so excessively that the coating is easily peeled off (this phenomenon is termed, "powdering" hereinafter) when the sheet is subjected to press forming work. On the other hand, there is the advantage that a steel sheet of stable quality can be produced using continuous anneal even at an ordinary coiling temperature of 600.degree.-650.degree. C.
In contrast, in the case of Nb killed steel, it is necessary to coil the hot rolled strip at a high temperature (coiling temperature .gtoreq.700.degree. C.). This is because when coiling is carried out at an ordinary coiling temperature, the complete recrystallization temperature becomes so very high that a semi-recrystallized portion remains when annealing is carried out within the temperature range feasible with a continuous annealing oven (not more than about 850.degree. C.) and, moreover, because in such case the quality varies greatly with the amount of Nb. It has frequently been reported that when coiling is carried out at a high temperature, it is possible to obtain a steel sheet having a high r value in all but the end portions of the hot rolled strip at an annealing temperature of about 800.degree.-850.degree. C. In the high temperature coiling, however, the formed scale becomes so thick as to impair the pickling efficiency. Moreover, as the rate of cooling at the coil ends is high, it becomes impossible to obtain a product of sufficient quality. For these reasons, there is a pronounced decline in product yield.
A very low carbon steel sheet with added Ti and Nb is disclosed in U.S. Pat. No. 3,765,874. The amount of Nb is more than 0.025%, and the steel sheet containing more than 0.025% Nb as a solid solution is disclosed in this patent.
The inventors have investigated the steel of the above composition in detail, and found that this steel sheet has the following defects.
As the recrystallization temperature is considerably high, a good quality cannot be obtained by the usual anneal temperature. Further, in the rapid heating and short time annealing, such as continuous anneal, and in the feasible annealing temperature (usually, less than 850.degree. C.), a satisfactory recrystallization will not take place, or the grain growth after recrystallization never occurs.
Since the amount of Nb added is too much: (1) Carbide thus precipitated is fine, so the migration of the grain boundary is greatly hindered by the precipitate; (2) the growth of recrystallized grains is considerably restricted by the solution drag effect due to the high amount of solid solution Nb. In other words, since the amount of Nb is too much, as described hereinbefore, it has the same defect as that of the Nb killed steel.
As a steel stock for the continuous anneal, a good r value and El cannot be obtained by low temperature coiling, and the stock is hard; the deterioration of quality of the coil end portion is still remarkable even by the high temperature coiling. Furthermore, as the steel contains to much of both Ti and Nb its chemical treatability is inferior.